kotlinlang #announcements

Hello guys, as a Kotlin library provider, is it better to always provide my framework at latest Kotlin version (i.e. 1.2) or stay a while on most stable ?

GauthierPLM

12/06/2017, 4:46 PM

I just randomly discovered kotlin Sequences (https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.sequences/index.html) while reading the AMA on Reddit. Is it mentioned in the doc (other than the reference) ? It would be nice to talk about it in the page of collection as it could be really useful. Also a tutorial on laziness would be nice. i think i could use it in my app, but don’t know yet how to do that.

joshua.ewer

12/06/2017, 7:19 PM

anyone have a good idiom for solving this problem?

given two nullable lists,
a: List<String>?
and
b:List<String>?

if any are not null, return a concat list

else return null

elizarov

12/06/2017, 7:29 PM

b?.let { a?.plus(it) ?: it } ?: a

to return null only if both are null

bj0

12/06/2017, 7:30 PM

?: it

joshua.ewer

12/06/2017, 7:48 PM

after writing a handful of tests i ended up going with leo’s suggestion since the logic did get lost in the ?.{}?.?{} soup. super helpful conversation though

jmfayard

12/06/2017, 7:58 PM

I have a Kotlin first world problem. After doing backend and Android in Kotlin, I don't know what I want to try out first: Kotlin react js, multiplatform or tornado FX?

jtonic

12/07/2017, 6:05 AM

Dear Kt community, Could you please emphasise why the following compilation error occurs.

Error:(13, 30) Kotlin: Type checking has run into a recursive problem. Easiest workaround: specify types of your declarations explicitly

in the following case

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package ro.jtonic.tutorials.kt.oop.miscs

private const val PI = Math.PI
const val E = Math.E

private fun computePi() = PI
fun computeE() = E

class MyMath {

    val E = FilePrivateUtils@::E.get() // here is the compilation error
    val pi = FilePrivateUtils@::PI.get()

    fun computingPi() = FilePrivateUtils@::computePi.invoke()
    fun computingE() = FilePrivateUtils@::computeE.invoke()
}

Thank you.

Ruckus

12/07/2017, 4:46 PM

@abreslav I've seen you mention several times that you think companion objects could have been done better, and in the AMA you said you may be able to fix that in the future. Is there any chance you could elaborate on what you see as shortcomings to the current implementation and how it might be improved?

cedric

12/07/2017, 6:42 PM

Given

data class Foo(var s: Int? = null)

, is it possible to invoke the default constructor by reflection without requiring

@JvmOverloads

? Without it, I see that this class has two constructors, one with a last parameter

DefaultConstructorMarker

but it doesn’t look like I can invoke it.

adams2

12/07/2017, 7:10 PM

does the initialization call my custom setter?

Thiago

12/07/2017, 8:02 PM

I did not understand.

emmanuelvinas

12/08/2017, 9:03 AM

Someone knows if the video of yesterday's webinar by @yole is available ?

michaelzinn

12/08/2017, 10:27 AM

For those who are stuck with a Java project that you can’t port to Kotlin I backported Kotlin’s null safety to Java (meta-annotation, ?. and ?:) : https://github.com/cosee/null4j

👍 1

michaelzinn

12/08/2017, 10:52 AM

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x?.let { bla(it) }

becomes

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let(x, Bla::bla)

for example.

karelpeeters

12/08/2017, 11:53 AM

Because it can also be used in

when

and

for

statements, where it has a slightly different meaning.

Thiago

12/08/2017, 2:19 PM

Helllo @agrosner I'm having the following error in my project: Error: error: * ========== * class com.raizlabs.android.dbflow.processor.TableValidator: Table Value must provide a visible, default constructor. * ========== * However, I checked the link https://github.com/Raizlabs/DBFlow/issues/1463 that for item 3, you suggest that 'Make sure you provide default values for all fields.' However, all fields in the data class begin with the value null. What would be the solution to this case? Would you help me? Os campos do data class sao : @Column @PrimaryKey(autoincrement = true) var id: Long? = null, @Column var example_value: String? = null, @Column var example_value: String? = null, @Column var example_value: String? = null, @Column var example_value: Int? = null, @Column var example_value: String? = null, @Column var example_values: String? = null, @Column @SerializedName("active") var isActive: Boolean? = null, @Column @Transient var example_value: Int? = null, @Expose var example_value: List<example_value>? = null)

stackoverflow 2

nickk

12/08/2017, 3:30 PM

I tried to overcome the JVM type erasure by storing the Type at compile time, like so:

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data class ResultWrapper<out T>(val request: RequestInfo, val obs: T, val typeOfResult : KClass<*>? = null)

However doing a

when

on the

typeOfResult

field does not work:

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when (item.typeOfResult) {
             is Enemy -> {
                  messages.add("Received instance of type ${item.typeOfResult.simpleName} !")
              }

Any ideas/suggestions?

evan

12/08/2017, 10:20 PM

This piece of code does not compile:

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val byteVal:Byte = 0
val isZero = byteVal == 0

You have to change it to this to make it compile (and run):

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val byteVal:Byte = 0
val isZero = byteVal == 0.toByte()

This seems overly complicated to just compare a byte-value with a literal. Do we really have to call a method (

toByte()

) to do this? Is there a modifier (much like

for longs and

for floats) to make this simpler and less expensive?

karelpeeters

12/09/2017, 8:57 PM

Because transforming it into a map only "works" when your map function is injective.

12/10/2017, 1:12 PM

Hi, is there a reason why the following class wouldn't compile?

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class Example {

    private val backingMap = mutableMapOf<String, MutableSet<String>>()

    fun addVertex(v: String): Boolean = backingMap.putIfAbsent(v, mutableSetOf()) == null
    
    fun addEdge(v: String, w: String): Boolean {
        listOf(v, w).map(::addVertex)
        return backingMap[v]!!.add(w)
    }

}

robfletcher

12/10/2017, 7:29 PM

I have 2 methods

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fun <ACTUAL> expect(
  actual: ACTUAL,
  block: ObjectAssert<ACTUAL>.(ACTUAL) -> Unit
): ObjectAssert<ACTUAL>

and

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fun <ELEMENT> expect(
  actual: List<ELEMENT>,
  block: SoftAssertionListAssert<ELEMENT>.(List<ELEMENT>) -> Unit
): SoftAssertionListAssert<ELEMENT>

that the compiler is unable to distinguish between at the call site. I get why that’s an issue but confusingly I have overloads of both that only accept the first parameter:

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fun <ACTUAL> expect(actual: ACTUAL): AbstractObjectAssert<*, ACTUAL>

and

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fun <ELEMENT> expect(actual: List<ELEMENT>): ListAssert<ELEMENT>

that the compiler can distinguish. Can anyone enlighten me as to why one is a problem and no the other? Is there any way I can disambiguate the 2 param form?

pakoito

12/10/2017, 8:41 PM

https://github.com/scala/scala/blob/v2.12.0/src/library/scala/collection/mutable/ArrayStack.scala#L1

Marcin Wisniowski

12/11/2017, 6:06 AM

I'm wondering if it's preferred to call a function when using

map

or pass a method reference:

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listOf("1", "2", "3").map { it.toInt() }
listOf("1", "2", "3").map(String::toInt)

Is either of these considered better? (Or am I overthinking this?)

karelpeeters

12/11/2017, 8:45 AM

Ah, gotcha, didn't see the

\n

str.split("\n").map { it.split("\t").map { it.toInt() } }

edwardwongtl

12/11/2017, 10:34 AM

Maybe you can also check #opensource

Slackbot

12/11/2017, 1:00 PM

This message was deleted.

lovis

12/11/2017, 1:11 PM

is there a way to configure "convert java to kotlin", to use

public

instead of

internal

when the visibility was

package-private

before? 🤔

EDesigner250

12/11/2017, 2:06 PM

can kotlin work with Spark

stkent

12/11/2017, 4:13 PM

Is there a way to perform a

fold

and access the intermediate values?